P2X4 receptors are one of the predominant subtypes of purinergic receptors expressed in macrophages and microglia and their up-regulation has been shown to contribute to neuropathic pain. P2X4 receptors are prominently localized to lysosomes and resist degradation by virtue of N-linked glycans decorating the intra-luminal loop of the receptor. In order to understand how the expression of these receptors at the plasma membrane is regulated, we compared the proportion of receptors expressed at the cell surface in cultured microglia and macrophages following exposure to modulators of microglial/macrophage activation. Surface expression was analysed by biotinylation of exposed proteins and by cross-linking proteins with membrane impermeant cross-linkers, followed by SDS-PAGE and western blotting. The modulators included lipopolysaccharide (LPS), ATP and phorbol esters. A 24h incubation with LPS (500ng/ml) resulted in an up-regulation of P2X4 surface expression in cultured microglia without an evident increase in total expression of the receptor, indicating redistribution from intracellular compartments to the plasma membrane. In contrast, similar exposure of cultured astrocytes to LPS had no effect on the surface expression of P2X4 receptors. Exposure of microglia to LPS was sufficient to inhibit proliferation and we have compared the involvement of the P2X7 receptor in both the anti-proliferative effects of LPS and the up-regulation of P2X4 receptors. Brief incubations with phorbol esters produced a similar up-regulation of surface P2X4 receptors in bone marrow derived macrophages (BMDMs) and we are examining the underlying mechanisms involved.